The invention relates to a method of measuring the pH value of a test solution with glass-electrode measuring cells and of simultaneously calibrating the measuring cells, in which method the potentials occurring at the pH glass electrode and the reference electrode are processed in a pH meter with temperature compensation and measuring-cell slope derivation.
Whereas the measuring-cell zero point of modern pH measuring cells remains largely stable, the measuring-cell slope has to be calibrated at certain intervals of time. For this purpose, the measuring cells are removed from the container, the continuous-flow vessel or pipe and calibrated externally in known buffers (calibrating solutions). In this process, the measuring-cell data, such as measuring-cell zero point and measuring-cell slope, are determined and their deviations compensated for in the pH meter. The calibration operation can often not be carried out at the processing temperatures but only at room temperature. For this reason, and also because of the costs associated therewith, a frequent calibration, for example to increase the measuring accuracy, generally has to be dispensed with.
It is furthermore known to withdraw the measuring cells pneumatically from the test solution and expose them to rinsing solutions and calibrating solutions. The deviations in the measuring-cell parameters found in this process are compensated for in the pH meter. Such systems are relatively complex and expensive. Problems are presented, in particular, by the wear associated with the pneumatic movement of the measuring cell and the satisfactory sealing of the measuring liquid, i.e. its retention in the container.